Electroluminescent devices are structures which emit light when subject to an applied electric field. In its simplest forms an electroluminescent device comprises a light-emissive layer between two electrodes. The cathode electrode injects negative charge carriers electrons) and the anode electrode injects positive charge carriers (holes) into the light-emissive layer. Light emission occurs when the electrons and holes combine in the light-emissive layer to generate photons. As a practical aspect, one of the electrodes is typically transparent, to allow the photons to escape the device. The light-emissive layer should be made from a light-emissive material which may be laid down as a film without substantially affecting the luminescent characteristics of the material and which is stable at the operational temperature of the device.
Organic electroluminescent devices which use an organic material as the light-emissive material are known in this art. Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminescence. U.S. Pat. No. 4,539,507 discloses the use of small molecule organic materials as the light-emissive material, such as 8-hydroxy quinoline (aluminium) “Alq”. PCT/W090/13148 discloses an electroluminescent device comprising a semiconducting layer comprising a polymer film as the light-emissive layer which comprises at least one conjugated polymer. In this case, the polymer film comprises a poly (para-phenylenevinylene) (PPV) film.
The color of the light generated by the light-emissive material is determined by the optical gap or bandgap of the organic light-emissive materials, that is to say the difference in energy levels between the “highest occupied molecular orbital” (“HOMO”) and the “lowest unoccupied molecular orbital” (“LUMO”) levels. Effectively, the bandgap is the energy difference between the valence and conduction bands. These levels can be estimated by photoemission measurements and measurements of the electrochemical potentials for oxidation and reduction. The level of these energies is affected by numerous factors. Accordingly, the use of such values is indicative rather than quantitative.
It is known to use a semiconductive conjugated copolymer as the light-emissive layer in an electroluminescent device. The semiconductive conjugated copolymer comprises at least two chemically different monomer units which, when existing in their individual homopolymer forms, typically have different semiconductor bandgaps. The proportion of the chemically different monomer units in the copolymer can be selected to control the semiconductor bandgap of the copolymer so as to control the optical properties of the copolymer. To some degree, the extent of conjugation of the copolymer can be said to affect the Π-Π* bandgap of the copolymer. Increasing the extent of conjugation has the effect of decreasing the bandgap up to the point of bandgap convergence. Therefore, selection of appropriate reaction components may be used to modulate the bandgap. This property may be exploited so that the semiconductor bandgap is modulated to control the wavelength (i.e. color) of radiation emitted during luminescence. This gives the very desirable feature of controlling the color of light output from the polymer. This property is useful particularly in the construction of electroluminescent devices.
EP 0686662 discloses a device for emitting green light. The anode is a layer of transparent indium-tin oxide. The cathode is a LiA1 layer. Between the electrodes is a light-emissive layer of PPV. The device comprises also a hole transport layer of polyethylene dioxythiophene which provides an intermediate energy level which aids the holes injected from the anode to reach the HOMO level in the PPV.
“Efficient Blue-light Emitting Devices From Conjugated Polymer Blends”, Burgeson et al., Adv. Mater. 1996, 8, No. 12, pages 982-985 describes a blue-light emitting device which employs conjugated polymer blends. The emissive layer of the dev ice consists of a blend of PDHPT with PDPP. Light emission is from the PDHPT alone.
Organic materials having smaller optical gaps, towards the red end of the visible spectrum, are of particular interest at present. It has been suggested that conjugated polymers that possess narrow bandgaps are a current topic of interest because such polymers will be useful not only in optical devices but are expected to be promising candidates for intrinsic organic conductors, non linear optical devices, solar cells and IR emitters and detectors.
However, few low bandgap polymer materials are known which show good optical device characteristics when used in an optical device. These characteristics include the quantum efficiency of the copolymer when excited to luminesce, the solubility and processability of the polymer and the lifetime of the polymer when used in a device. For electroluminescence, the quantum efficiency is defined as photons out per electron injected into the structure. Other relevant characteristics for consideration are stability of the polymer during use and storage of the device.
Low bandgap materials are not well known partly because they are difficult to make.
It may be noted that polymers made by electrochemical oxidative coupling usually are not suitable for use as emitters in an electroluminescent device. This is because they have poor device characteristics such as having a large number of defects and being substantially insoluble and not processable.
In Windle et al. J. Org. Chem., 1984, 49, 3382 and Kobayshi et al., J. Chem. Phys., 1985, 82, 5717 the reported discovery of polybenzo [c] thiophene whose bandgap is about 1 eV lower than that of polythiophene showed a possibility of tuning the bandgap by structural modification. Recently, efforts have been devoted theoretically and experimentally in order to explore the correlation between the structures and bandgaps of polymers and to further reduce bandgaps.
Several approaches to achieving a narrow bandgap have been suggested. One is the copolymerisation of aromatic and o-quinoid units, indicating that the combination of monomer segments with different electronic structures can lower the bandgap through the relaxation of bond-length alternation. This is discussed in Kurti et al, J. Am. Chem. Soc. 1991, 113, 9865.
Another approach is the alternation of strong electron-donating and electron-accepting moieties as disclosed in Havinger et al, Polym. Bull., 1992. 29, 119. This suggests that the mixing of monomer segments with higher HOMO and lower LUMO is effective to reduce the bandgap due to the intra chain charge transfer. The affect of steric interaction between adjacent units relating to coplanarity having regard to maximizing the effective conjugation length along the polymer backbone is addressed in Nayak et al, Macro molecules, 1990, 23, 2237.
Narrow bandgap systems symbolized as A-Q-An′ where A is a kind of aromatic-donor unit and Q is a kind of o-quinoid-acceptor unit are disclosed in “Design of Narrow-bandgap polymers”, Chem. Mater., 1996, 8, pages 570-578. The bandgaps determined from the polymers on ITO-coated glass electrodes vary from 0.5 to 1.4 eV. The authors conclude that these values are small compared with usual conjugated polymers, confirming that the polymers are narrow-band gap systems. Furthermore, the authors conclude that the results show that the bandgap is widely tunable by the polymer structure.